In
the above image, the top left corner
shows a ground-based Digitized
Sky Survey image of the full
field of NGC 300. An outline of
the Hubble Heritage ACS image is
marked and shown in the image at
top right. A detailed blow-up of
this image (at bottom) shows individual
stars in the galaxy. A background
spiral galaxy is visible in the
lower right corner.
Illustration Credit: NASA and Z.
Levay (STScI)
Image Credits: NASA, The Hubble
Heritage Project (STScI/AURA) and
Digitized
Sky Survey

Zoom
AnimationMPEG 5.5MBZoom from the Digital
Sky Survey NGC 300 and dissolve
into the Hubble's Advanced Camera
for Surveys image of a section of
NGC 300.

Animation
Credit: NASA, Z. Levay and G.
Bacon (STScI)

Double ASC
pointing of NGC 300
(Click to Enlarge)

Star forming regions in NGC 300

By Fabio Bresolin

Place cursor
over image
for a comparison of visible
light and Halpha.
Click to enlarge the Halpha
image.

Image
courtesy of F. Bresolin (IfA/U.
Hawaii)

NGC 300 is a very photogenic galaxy.
Here is a narrow-band Halpha image
taken by F. Bresolin at the ESO
2.2m telescope on La Silla, Chile.
Clusters of young (few million years
old) and massive (up to 100 solar
masses) stars form along the spiral
arms, and with their enormous output
of radiation they ionize the surrounding
clouds of gas, mostly made up of
hydrogen. The recombination of hydrogen
atoms back to the neutral state
produces photons of light at specific
wavelengths (emission lines). The
Halpha line, that is the emission
by hydrogen atoms at 6563 Angstroms
(1 Angstrom = 0.00000001 cm), is
particularly intense, and allows
us to trace the location of recent
star formation in spiral galaxies.
Astronomers call these sites HII
regions.

Besides being tracers of
massive stars, the analysis of the emission
line spectra of these regions of ionized
gas provides us with crucial information
on the chemical composition of spiral
galaxies.

Most of the stars visible
in this image do not belong to NGC 300,
but are instead foreground Milky Way objects.
The HST image shown in this release covers
the very central part of the galaxy, devoid
of large HII regions.

Blue supergiant stars as distance indicators

Quantitative stellar astronomy
can be useful for the determination of
extragalactic distances. A small team
of astronomers, composed by Rolf Peter
Kudritzki, Fabio Bresolin (Institute for
Astronomy, University of Hawaii) and Norbert
Przybilla (now at Bamberg University in
Germany), have recently discovered a simple
relation linking the intrinsic luminosity
of blue supergiants to their fundamental
parameters: surface gravity and temperature.
(Graph courtesy of F.
Bresolin (IfA/U. Hawaii))

This plot shows such a relationship,
based on observations of stars in NGC
300, NGC 3621 (another HST target galaxy;
for an image follow this link to the Araucaria
Project homepage) and a number of
galaxies in the Local Group, including
our own Milky Way. Each star analyzed
is represented by a dot, with different
symbols used for the different galaxies.
The y-axis represents the bolometric magnitude,
a measure of the stellar intrinsic luminosity
(smaller numbers corresponding to brighter
objects). The objects at the top of the
diagram, with bolometric magnitudes between
-9 and -10, are among the most luminous
normal stars known. Only supernovae and
certain members of a class of stars known
as Luminous Blue Variables exceed these
values of luminosity for a single star.
The x-axis is a simple combination of
the stellar gravity and effective temperature,
which are measured from spectra of the
stars. More extended (lower gravity) and/or
hotter objects are found to the right
in this plot.

This diagram shows a tight
correlation between luminosity and the
flux-weighted gravity, which can be used
to measure extragalactic distances. For
a given star it is relatively simple to
determine gravity and temperature (from
spectra), together with its apparent brightness
or magnitude (from images). The vertical
shift necessary to match the position
of the star in the plot with the calibrating
line (seen as dashed in the diagram) would
provide the distance modulus (a measure
of the distance). In practice, in order
to reduce the statistical uncertainties,
one needs data for a number of stars in
a given galaxy.This technique is currently
being tested as part of the Araucaria
Project.